Temerarious is a word that is only really at home in literary texts. It means ‘reckless’, ‘rash’ or ‘unreasonably adventurous’.
You may find in some historical texts that temerarious has been used to mean ‘haphazard’ or ‘happening at random’, but this usage is now obsolete.
The OED gives the first recorded usage as in 1532. But I think this is a good example of temerarious used well:
“The King was one of the first that entred [the breach], choosing rather to be thought temerarious then timorous.”
The word comes from the Latin temerarius , where temere means ‘rashly’. (The word temerity, meaning ‘excessive confidence’ or ‘audacity’, also has its roots in temere.) The suffix -ous denotes ‘full of’ or ‘characterised by’. The noun is temerariousness.
- The Oxford English Dictionary Online